To,

The Principal,
(YOUR SCHOOL NAME)
Subject : Application for sick leave
Sir,
                 Most Respectfully It is stated that I have been suffering from fever since last night.I went to Doctor for treatment and he said that I would rest for three days. Now i am under the treatment.So I request to you that leave me for three days. 
                   I shall be very thankful to you for this act of Kindness.
Dated 27,2,2016.
                            Your's Obediently
                             (Your Name)
                            Class: (Your Class)
          Roll Number: (Ur Roll No)